Acid rhodium plating



Patented June 4, 1946 ACID mmmUM PLATING Abner Brenner, Chevy Chase, and Walter A. Di-

son, Silver Spring, Md, asslgnors to the United States'of America, as rcpresentedby the Scoretary of Commerce No Drawing. Appli amen May 29, 1944,-

sci-inactivate 5 Claims. joiner-41) (Granted under the act of March 3, ms, as amended April so, 1928; 370 o. o. 151) The invention or discovery described herein may be made and used by or for the Government of the United States without the payment of any royalty therefor.

ins and aims generally to improve the same."

Rhodium plated coatings are hard and bright and have a high reflectivity and are thus useful for decorative purposes and particularly for reflecting surfaces. The high reflectivity of good rhodium coatings, 75% as compared to 60-65% for chromium coatings, and their chemical inertness and resistance to corrosion render them particularly desirable for searchlight reflector surfaces (see Shumpelt, Trans, of the Electro-' Chem. Soc., 80 p. 495, year 1941 at 554-7) However, a substantial premium is paid for the 15% increase in reflectivity thus obtained, since rhodium costs over $100 per ounce as compared to a few cents for chromium. Thus it is particularly desirable to realize the maximum reflectivity of the rhodium coating both from the standpoint of technical efficiency and that of economic justification.

Experience has shown that in using the usual rhodium plating baths the reflectivity of the deposit, with continued use of the plating solution, drops materially below the 74.5: 0.5% required by army specifications for searchlight reflector rhodium electroplate. Prior to the present research it was considered necessary, on such decrease of reflectivity, to rctum the plating solution to the platinum-metal refiner for recovery of the rhodium and reincorporation thereof in a new solution. The refining charge for remaking involves not only inconvenience and expense of transport, but also the loss of its use during the period required for shipment and remaking. The method of remaking employed by the reflner is not publicized, but is believed to involvethe addition of nitrites to convert the rhodium to a soluble complex, adjustment of the pH to precipitate all metals except the rhodium and recovery of the rhodium as a salt from which a new bath is made up.

The research disclosed herein has aimed to determine the cause of the decrease in reflectivity, and to discover a method applicable to the solutions themselves, and at the plating plant,

if desired, to solve the problem or reflectivity loss.

Typical rhodium plating baths comprise solutions of rhodium Phosphate or sulfate, or solutions of rhodium phosphate containing some sulfuric acid. Baser metals may become intro- I per liter (see Shumpelt cited above). The method duced into the solutions accidentally from metal objects falling into the bath, by action of the bath on unplated recesses of objects being. .plated, from bus bars, or from tank linings or insulating materials. Many of the solutions are proprietary and their exact constitution is not generally or publicly known. Moreover, in the case of the phosphate-containing solutions the exact state of the combination of the rhodium is unknown, since there seem to exist a number of rhodium phosphates of a more or less complex nature to which no definite formulas can as yet be assigned. The solution in general, however. may be regarded as comprising from about 1 to about 2 grams of rhodium metal per liter, and the content of sulfuric acid in solutions containing it usually runs from about 30 to 40 grams of bath treatment to eliminate the decrease o. reflectivity of the coatings which has resulted from the present research is applicable to all ity is slight, say only a few percent, it is observed with a reflectometer, thus determining the loss in reflective efliclency. If the concentration of deleterious metals is high. the deposit may be visibly darkened and streaked as observed by the naked eye.

Accordingly, the present research has been di- 7 rooted to the discovery of a method of removing zinc and like deleterious metal from the bath,

without removing rhodium, and without render-v dicator.

3 2. Subordinate to this discovery are ancillary improvements in the procedure. For example, the standard test for ferrocyanides by addition of uranium compounds was found not usable in the acid rhodium plating solutions. It ha accordingly been necessary to devise a means of determining the proper amount of ferrocyanide to add. In the present method this determination is made by an outside indicator consisting of an acidified ferric solution.

3. It has further been found that while it is desirable to add a slight excess of ferrocyanide, to insure full precipitation of the deleterious metals, and that while this can be done without precipitating the rhodium, the presence of such excess of ferrocyanide is harmful to the operation of the bath and may even inhibit the deposition of rhodium. Provision is accordingly made for subsequent removal of the excess ferrocyanide when such is added, as by the addition of a soluble ferric salt, preferably in slight excess, and it has been determined that the slight excess of iron so added does not decrease the reflectivity of the rhodium deposit.

4. In practicing the present invention with any given bath, it is expedient to predetermine the amount of ferrocyanide to be added by suitable titration. For example, in employing potassium ferrocyanide, about .250 milliliters of the bath maybe titrated with a solution that contains say, 20 grams per liter (20 g/l) of K4Fe(CN) 8.3H2O, by adding the ferrocyanide solution until a slight eX- cess is present.

4a. If iron is absent from the bath, this excess can be detected by using on a spot plate an acidified solution of ferric sulfate as an outside indicator. A blue color indicates an excess of ferrocyanide.

4b. If the bath contains iron, the blue color of the precipitate in the sample being titrated will make it difiicult to use the spot plate test. The test is then conveniently made on a filtered portion of the titrated test solution; or a drop of the titrated solution may be placed on a sheet of filter paper with a drop of ferric sulfate indicator placed nearby. As the drops spread out they will meet and at the line of contact a faint blue line will form if an excess of ferrocyanide is present.

40. By calculation, based on the titration of the sample, the proper amount of a ferrocyanide solution is then determined'and added to the plating bath to precipitate all the harmful metals.

5. As above mentioned, any excess of ferrocyanide is harmful to the operation of the bath and may even inhibit rhodium deposition. Accordingly, in the preferred embodiment of this invention the slight excess of ferrocyanide which has been added is removed by addition of a soluble ferric salt, preferably ferric sulfate. Enough of this is added to the bath to precipitate the excess ferrocyanide and leave a slight excess of iron, e. g., 5 or milligrams per liter, this being a concentration that can readily be detected with a potassium ferrocyanide solution as an outside in- This amount of iron does not decrease the reflectivity 'of the rhodium deposit.

6. After the addition of the ferrocyanide, followed by that of the iron, the precipitates are removed. This may be accomplished by immediate filtration using a filter aid, as diatomaceous earth, to facilitate removal of the finely divided precipitate; or the bath may be allowed to stand over night, preferably warm (i. e. at about 50 degrees to 60 degrees C.), to coagulate the precipitate to facilitate filtration 7. If certain inorganic contaminants are present in the bath such contaminants may be removed by the well known method of absorption on activated charcoal.

8. The metallic impurities which can be removed from the rhodium bath by means of ferrocyanide are those which form ferrocyanides insoluble in dilute acid; Among these metals are lead, mercury, silver, tin, cadmium, zinc, iron, cobalt and nickel. Copper ferrocyanide is somewhat soluble in dilute acids so that copper would not be primarily precipitated unless a considerable quantity were present, or unless the acidity of the solution were reduced. However, some copper is carried down, even in the acid bath, by the other metals undergoing precipitatiton. Rhodium in the concentration present in th platin bath is not precipitated by ferrocyanide.

9. While the employment of potassium ferroeyanide and ferric sulfate are preferred, soluble analogues thereof are contemplated in the broader aspects of the invention. Thus the alkali metals and the alkali earth metals such as calcium and strontium, having soluble ferrocyanides may be used; and soluble ferric salts of acids which are ordinarily neither oxidizing nor reducing such as the chloride, or sulfamate may be employed in lieu' of the ferric sulfate without departing from the broad invention. Further, the employment of strontium ferrocyanide has the advantage in the acid. baths that the strontium ion will also be removed as insoluble strontium sulfate at the same time the ferrocyanides of the harmful metals are being precipitated.

Thus in this case the precipitate will include both ions of the reagent as well as the harmful metals.

10. Similarly various modifications of procedure may be adopted to suit particular baths being rejuvenated by the new method. For example, if desired, the precipitated metals may be filtered out before addition of the precipitant for excess ferrocyanide, followed after such addition, by filtration of the latter precipitate; and other modifications may be made. Thus the invention is notlimited to the particular procedural and ancillary steps described.

What is claimed is:

l. The method of treating a rhodium plating solution to maintain high reflectivity of rhodium coatings electrodeposited therefrom, which consists in removing from the plating solution, when the reflectivity of such coatings tends to diminish, zinc and like base metals the presence of which is deleterious to such reflectivity, such removal being effected by adding to the plating solution a solution of alkali metal ferrocyanide, separating the solution from the resulting precipitate, and then returning the separated solution for further electrodepositi'on of rhodium coatings therefrom. I

2. The method of treating a rhodium plating solution to maintain high reflectivity of rhodium coatings electrodeposited therefrom, which consists in removing from the plating solution, when the reflectivity of such coatings tends to diminish, zinc and like base metals the presence of which is deleterious to such reflectivity, such removal being effected by addingto the plating solution a solution of alkali metal ferrocyanide in slight excess, removing the excess ferrocyanide by addition of a soluble ferric salt of an acid which is ordinarily neither oxidizing nor reducing, separating the solution from the precipitates, and then returning the separated solution for further electrodeposition of rhodium coatings therefrom.

aaonssr 3. The method of treating a rhodium plating solution to maintain high reflectivity of rhodium coatings electrodepositel therefrom, which consists in removing from the plating solution, when the reflectivity of such coatings tends to diminish, zinc and like base metals the presence of which is deleterious to such reflectivity, such removal being efiected by adding to the plating solution a reagent selected from the group consisting of the soluble ferrocyanides of the alkali metals and alkali earth metals, separating the solution from the resulting precipitate, and then returning the separated solution for further electrodeposition of rhodium coatings therefrom.

4. The method of treating a rhodium plating solution to maintain high reflectivity of rhodium coatings electrodeposited therefrom, which consists in removing from the plating solution, when the reflectivity of such coatings tends to diminish, zinc and like base metals the presence of which is deleterious to such reflectivity, such removal being efiected by adding to the plating solution in slight excess a reagent selected from the group consisting of the soluble ferrocyanldes of the alkali metals and alkali earth metals, removing 6 v v the excess ferrocyanide by addition of a soluble ferric salt of an acid which'is not ordinarily oxidizing or reducing, separating the solution from the precipitates, and then returning the separated solution for further electrodeposition of rhodium coatings therefrom.

5. The method of treating a, rhodium plating solution to maintain high reflectivity of rhodium coatings electrodeposited therefrom, which consists in removing from the plating solution, when the reflectivity of such coatings tends to diminish, zinc and like base metals the presence of which is deleterious to such reflectivity, such removal being effected byadding to the plating solution a solution of potassium ferrocyanide in slight excess of the. amountnecessary to precipitate such metals as ferrocyanides, removing the excess ferrocyanide by addition of ferric sulfate, separating the solution from the resulting precipitates,

and then returning the separated solution for further electrodeposition of rhodium coatings therefrom.

' ABNER BRENNER.

WALTER A. OLSON.v 

